Ultrasonic surgical instruments are useful surgical instruments for performing certain medical and surgical procedures. Generally, an ultrasonic surgical tool includes a handpiece that contains at least one piezoelectric driver. A tip is mechanically coupled to the driver and extends forward from the housing or shell in which the driver is disposed. The tip has a head. The head is provided with features, often teeth, dimensioned to accomplish a specific medical/surgical task. An ultrasonic tool system also includes a control console. The control console supplies an AC drive signal to the driver. Upon the application of the drive signal to the driver, the driver cyclically expands and contracts. The expansion/contraction of the driver induces a like movement in the tip and more, particularly, the head of the tip. When the tip so moves, the tip is considered to be vibrating. The vibrating head of the tip is applied against tissue to perform a specific surgical or medical task. For example, some tip heads are applied against hard tissue. One form of hard tissue is bone. When this type of tip head is vibrated, the back and forth vibrations of the tip teeth, saw, remove, the adjacent hard tissue. Still other tip heads are designed to be placed against soft tissue. Some ultrasonic tools also remove tissue by inducing cavitation in the tissue and surrounding fluid. Cavitation occurs as a result of the tip head moving back and forth. Specifically, as a result of these vibrations, small voids, cavities, form in the tissue and surrounding fluid. These cavities are small zones of extremely low pressure. A pressure differential develops between the cells forming the tissue and these cavities. Owing to the relatively large magnitude of this pressure differential, the cell walls burst. The bursting of these cell walls, removes, ablates, the cells forming the tissue.
The head of an ultrasonic tip is often relatively small. Some heads have diameters of less than 1.0 cm. An ultrasonic tool essentially only removes the tissue adjacent to where the head is applied. Owing to the relative small surface area of their heads, ultrasonic handpieces have proven to be useful tools for precisely removing both hard and soft tissue.
Most tips are designed so that when the drive signal is applied, the tip head vibrates in a single mode. Here the vibration mode is understood to be the path of travel along which the tip head travels. The majority of tips are designed to vibrate linearly. This means the heads move back and forth along an axis that is essentially in line with the proximal-to-distal longitudinal axis along the tip. Some tips are designed so that their heads, when vibrated, engage in a torsional or rotation vibration. This means that that head, when excited into vibration, rotates around the tip longitudinal axis. Still other tips are designed to flex. This means that when the tip is excited, the longitudinal axis of the tip bends back and forth. The tip head moves with the bending, the flexing, of the tip.
Problems can arise when a tip head only vibrates longitudinally. This is because this type of tip head movement frequently induces cavitation in the tissue along the tip shaft. This can be a problem when the tip is used to remove hard tissue, bone, in close proximity to soft tissue that should not be subjected to removal. Types of soft tissue that should not be removed included both blood vessels and tissue that is part of the nervous system. The problem occurs because the cavitation can result in the unwanted removal of this soft tissue.
Tips are now available that reduce this unwanted cavitation. These tips are designed to vibrate in two modes. The tip vibrates longitudinally. The tip also vibrates torsionally, around the longitudinal axis of the tip shaft. One such tip is the Long Micro Claw tip available from the Applicant, Stryker Corporation, of Kalamazoo, Mich. The structure of this tip is disclosed in U.S. Pat. No. 6,955,680, COUPLING VIBRATION ULTRASONIC HAND PIECE, the contents of which is explicitly incorporated by reference.
When a drive signal is applied to a tip capable of vibrations in different modes, the tip head undergoes a movement that is the sum of the vibratory displacements. The head of a tip capable of simultaneous longitudinal and torsional vibrations when driven, simultaneously oscillates longitudinally and rotationally. FIG. 1 depicts this movements at a point on the tip head. As a result of these simultaneous vibrations, a point on the tip head moves back and forth along a section of helix. This movement is thus proximally and distally along the longitudinal axis of the head and rotationally around the longitudinal axis.
An advantage of so vibrating the tip is that the extent the tip shaft vibrates longitudinally is reduced. This results in a like reduction in the unwanted removal of tissue adjacent the shaft.
While the above ultrasonic tool system is useful, it is not without some disadvantages. One disadvantage is that, for this system to function, the two modes of vibration have to occur at the same frequency. This requires the tip to be especially designed to vibrate in this mode. This constrains the tips to certain sizes and shapes. This can make it difficult to provide tips able to be applied to sites in order to perform certain tissue removal procedures. Further, having to design a tip to this requirement can make the tip relatively expensive to produce.
Further, when a tip head undergoes this type of movement, an individual tooth on the tip head moves back and forth on a section of helix. This movement is over a track of typically less than 300 microns in length. In practice the movement of a single tooth is along a line that is diagonal to the longitudinal axis of the tip shaft. When an individual tooth cuts into bone, the tooth forms a groove that is diagonal to this axis. The back and forth motion of the tooth in a groove places a resistance against the tip that inhibits the motion of the head other than along the directions of the groove. This resistance can be appreciable because each tooth travels in its own groove. This inhibits the ability of the practitioner to steer, position, the tip in the desired direction.
Moreover, as a result of any cutting operation, the cut material forms debris in the vicinity of the tool performing the cutting. This applies to situations when an ultrasonic surgical tool is used to remove tissue. When the teeth of an ultrasonic surgical tool move back and forth in a linear path of travel the debris tend to accumulate between the teeth. The accumulation of these debris adversely affects the ability of the teeth to dig in and remove tissue.